During recent years in the dental field, cements which are formed from an inorganic powdery component and an organic polymer comprising carboxylic acid groups have achieved increased importance as filling materials for teeth as well as for fixing materials.
Carboxylate cements formed from zinc oxide and an aqueous polyacrylic acid solution are known, for example, from German Auslegeschrift No. 16 17 688. Although they are physiologically compatible, they are not suitable as permanent filling materials for aesthetic reasons where they are visible. Further, they show only moderate mechanical strength. Advantageous, however, is that they are chemically bound to the tooth enamel and to dentin. Thus carboxylate cements are broadly used as linings in a tooth cavity and as fixing cements.
It is known from Czechoslovakian certificate of authorship, No. 156 804, that the mechanical strength of the above described cements which are formed from zinc oxide or a mixture of zinc oxide and magnesium oxide may be improved by using an aqueous solution of a copolymer of acrylic acid with 0.1 to 50% of another unsaturated carboxylic acid or an anhydride thereof as a mixing component. While the thus formed cements show improved strength, the aesthetic drawbacks still are the same as those of a conventional zinc oxide polyacrylic acid system.
Permanent filling materials for dental use which show an excellent aesthetic effect together with high strength are the so-called silicate cements which are formed from a silicate glass powder and an aqueous solution of phosphoric acid.
A disadvantage of the silicate cements is that they are harmful to the dental pulp and are relatively soluble under oral conditions. The former weakness is particularly serious; it necessitates the application of cavity lining agents, procedures which are time consuming and in many cases unreliable.
Compared with the above-described silicate cements a remarkable improvement was achieved by the development of the so-called glass ionomer cements, which not only have favourable aesthetic and mechanical characteristics, but also are satisfying concerning their physiological compatibility. A cement system of that type is described in German Offenlegungsschrift No. 21 01 889. By using aqueous solutions of polymers derived from unsaturated .alpha.,.beta.-dicarboxylic acids having four or five carbon atoms as a mixing liquid there is obtained a dental cement which shows an excellent setting performance, good strength, a good physiological compatability and a satisfying aesthetic appearance.
A surview of this field is provided in the article of A. D. Wilson in "Chemical Society Reviews", 7 (2), p. 265 to 296 (1978).
Still further cement systems having a powder component consisting of a metal oxide powder, such as zinc oxide, or a silicate cement powder, are known from German Auslegeschrift No. 24 39 882. As a mixing liquid for these cements there are used aqueous solutions of acrylic acid-itaconic acid-copolymers in which the preferred ratio of acrylic acid units to itaconic acid units is in the range of 91:1 to 2:1. Further, British Pat. No. 1,382,881 describes glass ionomer cement systems having a mixing component consisting of an aqueous solution of copolymers of unsaturated .alpha.,.beta.-dicarbocylic acids, which contain more than 50 mole % of unsaturated .alpha.,.beta.-dicarbocylic acid units. As specific examples for polymers in this patent maleic acid-itaconic acid-copolymers and copolymers of maleic acid and acrylic or methacrylic acid, comprising more than 90% of maleic acid units are stated.
In spite of their above described aesthetic and physiological advantages these cements still need to be improved, because their adhesion to the tooth enamel and to dentin is still insufficient. While a better adhesion could be achieved by the use of an aqueous polyacrylic acid solution as a mixing component, in such a case, however, the favourable setting characteristics of the cements would be lost.